Coherent optical communication systems provide a means of communication over an optical fiber channel with improved power and spectral efficiency. In such systems, an optical signal is transmitted from a transmitter and is received by a receiver over an optical fiber communication channel. At the receiver, the received optical signal or its portion thereof is mixed with a local oscillator (LO) laser through a 90° optical hybrid, and the composite optical signal is converted to an electrical signal through a set of photodetectors. The respective analog electrical signal at the output of each photodetector is sampled and digitized by a high-speed analog-to-digital converter (ADC), and subsequently processed in a joint fashion by a digital signal processing (DSP) circuit to detect the transmitted information data.
Coherent optical communication systems enable higher spectral and power efficiency by modulating information data onto both of carrier phase and amplitude. To reduce cost, a coherent receiver generally employs intradyne coherent detection where the receiver relies on DSP based carrier recovery or carrier phase and frequency estimation by eliminating the need for an optical phase-locked loop (PLL). For such cases, a coherent receiver uses a free running LO laser that is tuned to a nominal frequency and not phase-locked to the transmitter laser, thus giving rise to a frequency offset between transmitter and LO lasers. This frequency offset is known as local oscillator frequency offset (LOFO) and can be as high as ±5 GHz for commercial tunable lasers. However, it is well-known in the art that various power efficient solutions for carrier recovery require a limited amount of LOFO for proper detection of the transmitted information data. For example, if the frequency offset between transmitter and LO lasers is larger than 800 MHz or so, some of the well-known carrier recovery algorithms fail to lock.
For coherent systems without an optical PLL at the receiver and commercial lasers without strict requirements on their frequency offsets, there may be a need for a coarse estimation of the frequency offset between transmitter and LO lasers. One known carrier frequency recovery method includes using a phase detector to estimate carrier phase and a phase rotator to apply a phase rotation to the received symbols. Phase errors are detected between the rotated symbols and the corresponding recovered symbols.
In certain deployments of coherent optical systems, it is not possible to change the structure and format of the transmit data frame to accomplish a header or unique word based estimation of the LOFO. In addition, the transmit frame header or transmit unique word may not be known to the receiver due to proprietary nature of transmit data frame format among different manufacturers of the optical network equipment. Furthermore, the use of frame header or inclusion of unique word in the transmit data frame for frequency offset estimation may incur some penalty on spectrum efficiency of the system and may not be always affordable. To circumvent these issues, a blind method of LOFO estimation is desired that is independent of the transmit frame header and/or unique word in the transmit frame.
As such, a method and apparatus that addresses at least some of these problems is desired.